The present invention relates to an apparatus for accurately measuring the quantity of liquid in a tank. More particularly, the present invention relates to an apparatus for performing a ratiometric measurement of the liquid quantity in a tank, which apparatus is insensitive to sensor characteristics and excitation voltage and also produces a zero output when the sensed liquid quantity is zero.
Many fluid-gauging systems are known which provide an indication of the quantity of fuel in a fuel tank, and, more particularly, a fuel tank of an aircraft. In known fluid-gauging systems, the level of fluid in the tank is determined by sensing changes in the value of a capacitor located within the tank. The capacitor has two plates separated by an air-gap which is filled or emptied as the level of fluid within the tank changes. The filling or emptying of the air-gap alters the capacitance of the capacitor, thus providing an indication of the level of fuel within the tank. In such known systems, an alternating electric signal is supplied to one plate of the capacitor and the output from the other capacitor plate is provided to a suitable measuring circuit. It is also known to rectify the output signal of the capacitor to provide a sensed signal which is relatively immune to cabling effects. A known method of rectifying the capacitor output signal is to provide a pair of diodes (of opposite polarity) coupled to and in parallel circuit with the capacitor output electrode.
However, difficulties have been experienced with such known systems in that they are extremely sensitive to diode characteristics. Furthermore, any changes in excitation voltage radically alter the accuracy of the capacitor output signal. Finally, such known systems are incapable of providing a true zero output signal when there is no fuel in the tank.
One such known system is described in U.S. Pat. No. 4,259,865 to Myers. Myers discloses a capacitor-sensor located within the fuel tank and coupled to a diode pair for rectifying the capacitor output signal. The rectified signal is then provided to a measuring circuit which includes an operational amplifier driving a display device. Myers addresses the problem of compensating the output signal for variations in the excitation signal, temperature changes within the fluid, and variations in permittivity of the fluid. FIG. 2 of Myers is directed to an apparatus which compensates for temperature variations within the fluid. In FIG. 2, first and second capacitors are provided in the same fuel tank, each coupled to a rectifying diode pair. However, the rectified signal from the first diode pair drives the display device while the rectified signal from the second pair is fed back to the oscillator in order to compensate for variations in the excitation signal. Such a system is extremely sensitive to diode characteristics and will not produce a true zero output signal when the sensed quantity of fuel is zero.
Likewise, FIG. 3 of Myers discloses three capacitors within the fuel tank, each coupled to a rectifying pair of diodes. In FIG. 3, the third capacitor is coupled to a correction circuit which in turn is coupled to the display device and ensures that a true zero output is provided when the quantity of fuel in the tank is zero. However, this device is also extremely sensitive to diode characteristics. Furthermore, a person of ordinary skill in this field readily understands that the apparatus according to FIG. 3 of Myers is complicated, bulky and presents a real reliability risk.
Another example of a known fluid-gauging system is disclosed in U.S. Pat. No. 4,020,691 to Franklin. In Franklin, a single capacitor is located within each fuel tank and is coupled to a corresponding diode pair. One diode from each diode pair is coupled to a switching device which presents a display device with information from whichever tank's capacitor is coupled through the switching device. This display provides an indication of the fuel in each individual tank. The second diode in each diode pair is coupled to a sum circuit which drives a display showing the total quantity of fuel available in all fuel tanks. However, the system of Franklin is subject to the problems of sensitivity to diode characteristics, extreme sensitivity to variations in the excitation signal, and a failure to produce a true zero output when the sensed quantity of fuel is zero.
Other known fluid-gauging systems are disclosed in patents such as, inter alia, U.S. Pat. Nos. 4,583,402 to Myers et al, and 4,145,927 to Larson. Likewise, such known fluid-gauging systems suffer from the problems discussed above.
Therefore, what is needed is a simple, streamlined apparatus capable of reliably, yet accurately indicating the quantity of fuel in a tank without being subject to diode variations, excitation signal changes, and which produces a true zero output when a sensed quantity of fuel in the tank is zero.